This invention relates, in general, to semiconductor devices and, more particularly, to a monolithic temperature compensated voltage reference diode and method of integration into a semiconductor device to achieve a temperature compensated sustaining voltage and high energy sustaining capabilities.
In the past, several schemes have been used to protect semiconductor devices from potentially destructive voltages and currents. Such conditions are commonly encountered in the application of power semiconductor devices. For example, a power semiconductor device such as a power MOSFET, is frequently used to switch inductive loads. When the power MOSFET is switched off, the energy stored in the inductor will force the drain voltage of the power MOSFET to rise rapidly above the supply voltage. If no limiting means are employed, this rise will continue until the drain-source avalanche voltage of the power MOSFET is reached, whereupon the energy stored in the inductor will be dissipated in the power MOSFET during device avalanche. Such dissipation can cause avalanche stress-induced failure of the power MOSFET.
A more manageable form of stress commonly occurs in the operation of power semiconductor devices as the device switches current on and off within its normal mode of conduction. Such operation of a power MOSFET occurs when the current in the power MOSFET channel region is, and remains, under the control of the power MOSFET gate. In this state, the device conduction stress can be regulated by appropriately modulating the signal on the power MOSFET gate. It is understood in the art of power MOSFET design and processing that device avalanche stress is more potentially destructive than device conduction stress.
Various processing techniques are commonly employed to render the internal parasitic elements of a power MOSFET less susceptible to avalanche-stress induced failure. A problem with these techniques is that normal variations in the processing parameters of a power MOSFET may inhibit optimization or reduce the effectiveness of these techniques.
Other methods of protection involve the application of external devices to render the power MOSFET less susceptible to avalanche stress. One such method involves using a drain-source clamp diode: an external diode connected between the drain and source of the power MOSFET, whose avalanche voltage is less than that of the power MOSFET. When the rising drain-source voltage reaches the avalanche voltage of the drain-source clamp diode, the energy stored in the inductor is dissipated in the drain-source clamp diode rather than the power MOSFET. The amount of energy than can be safely dissipated in this fashion depends on the dissipation capability of the drain-source clamp diode--large amounts of energy require large clamp diodes. While the drain-source clamp diode is dissipating the inductive energy, the power MOSFET is idle.
A more advantageous method of protection involves diverting a small fraction of the inductive energy to the power MOSFET gate by means of a drain-gate clamp diode whose avalanche voltage is about two to three volts less than the avalanche voltage of the power MOSFET. A suitable gate-source termination resistor is also employed in this method. When the rising drain voltage reaches the avalanche voltage of the drain-gate clamp diode, the resulting avalanche current develops a voltage across the gate-source termination resistor which turns on the power MOSFET, effectively clamping its drain to the sum of the drain-gate diode avalanche voltage and the voltage across the gate-source termination resistor. In this method the power MOSFET acts as its own clamp, and dissipates the inductive energy in the less stressful conduction mode. It is customary to add a second blocking diode in back-to-back configuration with the drain-gate clamp diode to enable the gate-source voltage in normal operation to exceed the drain-source voltage.
An advantage of using a drain-gate clamp over using a drain-source clamp is that the drain-gate diode, blocking diode, and gate-source termination resistor handle only enough energy to charge the power MOSFET input capacitances and therefore may be small in size and cost.
A disadvantage of these external clamp methods is that additional parts are employed to protect the power MOSFET, thus increasing the cost of the total system. In addition, the physical layout of some applications may preclude placing the clamp circuitry in close proximity to the power MOSFET. The resulting parasitic inductances act as impedances that slow the response time of the clamp circuitry. Therefore the power MOSFET may have to endure some avalanche stress until the clamps become active. It would be advantageous to provide a means of protecting the power MOSFET that achieves intimate proximity to the power MOSFET and does not increase the number of additional system components.
An additional problem that is difficult to control is the change in drain-to-source avalanche voltage of a semiconductor device with device junction temperature. For example, the drain-to-source avalanche voltage temperature coefficient of a 100 V power MOSFET is about 9 mV per degree Celsius. Additionally, wafer fabrication variations may result in an avalanche voltage spread of several volts for different wafer fabrication lots. While such variations are not a problem in many applications, some applications can require a tighter avalanche voltage distribution with less variation over temperature.
If a drain-gate clamp diode is selected to be a temperature compensated zener diode, the sustaining voltage of the power MOSFET, being clamped to that of the drain-gate clamp diode, will, to a great extent, also be temperature compensated. In the industry, temperature compensated zener diodes can be formed by arranging two diodes in back-to-back configuration, one with a zener avalanche voltage of about 5.5 volts and the other forward biased when the first is reverse biased. The zener diode has a positive temperature coefficient (TC) and the forward biased diode has a negative temperature coefficient. To achieve a temperature compensated voltage reference for the back-to-back configuration, the positive temperature coefficient of the zener diode must be approximately equal in magnitude to that of the negative temperature coefficient of the forward-biased junction. It is well known in the art that a zener diode of about 5.5 volts has a positive temperature coefficient approximately equal in magnitude to that of the negative temperature coefficient of a forward-biased junction. The resulting avalanche voltage of the two diode back-to-back "zero TC" configuration is about 6.2 volts.
Higher sustaining voltage power MOSFETs with temperature compensation could be achieved by connecting a plurality of external zero TC zener diodes in series for use as a drain-gate clamp diode. Such a scheme would provide protection of the power MOSFET from avalanche stress and would provide a temperature compensated sustaining voltage, however, to do so would be cost prohibitive because it would greatly increase the system component count. In addition, the number of zero TC diodes may be limited by the physical layout of the system.
By now, it should be appreciated that it would be advantageous to provide an improved method of protecting a semiconductor device from avalanche stress which also provides a temperature compensated sustaining voltage of the semiconductor device.
Accordingly, it is an object of the present invention to provide a high voltage monolithic temperature compensated voltage reference diode which can be used to protect a semiconductor device and provide a temperature compensated sustaining voltage of the semiconductor device.
Another object of the present invention is to provide an integrated semiconductor device having an improved protection scheme and temperature compensated sustaining voltage.
A further object of the present invention is to provide a self-protecting, integrated semiconductor device which also exhibits a temperature compensated sustaining voltage.
An additional object of the invention is to provide a semiconductor device with a defined temperature coefficient of sustaining voltage.